Fiberoptic telecommunications are continually subject to demands for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM). With a DWDM system many different and separate data streams may concurrently exist in a single optical fiber. Each data stream represents a different channel within the optical fiber, where each channel exists at a different channel wavelength. The modulated output beam of a laser operating at the desired channel wavelength creates the data stream. Multiple lasers are used to create multiple data streams, and these data streams are combined onto a single fiber for transmission in their respective channels.
The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers.
With the requirement for multiple tightly spaced channels, stable control over the laser source's output frequency is important to system effectiveness. The lasers used in DWDM systems typically have been based on distributed feedback (DFB) lasers operating with a reference etalon that defines the ITU wavelength grid. Due to manufacturing as well as performance limitations, DFB lasers are used as single channel lasers, or as lasers limited to tuning among a small number of adjacent channels. As a result, DWDM applications would require multiple different DFB lasers each at a different channel wavelength.
Continuously tunable external cavity lasers have been developed to overcome the limitations of DFB lasers. These lasers have a laser source and reflective element that define an external cavity, where this external cavity is used for wavelength tuning. For example, a tuning element within the external cavity, such as, an etalon device, is mounted to a support that is fixed to the platform between the laser source and reflective element. Controlling the temperature of the platform tunes the laser by altering the optical path length of the external cavity. Separate tuning between grid wavelengths is achieved by separately tuning the tuning element.
While continuously tunable lasers are desirable over DFB lasers for certain applications, assembling these lasers is a time consuming process. Each component in the external cavity interacting with the laser beam is carefully adjusted for optimum performance. In the optical domain, this means that each component must be optically aligned so that they work in concert. Optical alignment is quite difficult, however, as optical alignment is dynamic where the mere act of ‘aligning’ one component can misalign another component. Currently, optical alignment is performed actively, where a component is positioned in the optical path of a laser beam, e.g., in the external cavity, and the resulting effect of that component on the laser beam is measured. With a spectrum analyzer, a component may be positioned and repositioned in a laser device until the desired effect of that component on the overall output laser beam is achieved. Then the next component is actively optimized. This process is time consuming and requires expensive instrumentation and complicated manual or automated algorithms to complete. The process is inefficient as well. With dual etalon tuners, each etalon is separately and actively aligned with the laser, adding to overall alignment time.